In recent years, heat power plants and atomic power plants have had increased capacity and, as a result, a turbine rotor which is employed in a steam turbine has also been increased in diameter. In the low-pressure turbine, in particular, which operates at low temperature and low pressure and which is large in specific volume, a so-called "shrinkage-fitted rotor" prepared by shrink-fitting discs onto a shaft is adopted as the rotor from the standpoint of manufacture of the base materials.
In particular, since, in an atomic power plant, the steam produced in a nuclear reactor is low in temperature and in pressure as compared with that in case of the heat power plant, and since, as a result, the volumetric flow rate per a unit output thereof is four to five times as large as that of the heat power plant, the above-mentioned shrinkage-fitted rotor has hitherto been widely employed in the field.
When the disc is shrink-fitted onto the shaft, a sufficient coupling for shrinkage-fit is provided between both the disc and the shaft and, at the same time, a key is provided therebetween, for the purpose of preventing slippage from taking place therebetween due to the torque imparted to moving vanes mounted onto the outer periphery of the disc.
During the operation of the steam turbine, however, wet steam is carried into a key groove and is condensed within the same. When the turbine operation is continued under the condition wherein the water exists in the key groove, it is likely that a crack is produced.
This crack is what is called "stress/corrosion cracking" which occurs when tensile stress is caused to act on a material in the presence of water. It is known that stress/corrosion cracking occurs when three factors--the properties of the material, environmental conditions such as ambient temperature and steam, and operational stress--have satisfied a specified condition. Since the shrinkage-fitted rotor is structured by shrinkage fit, a tensile stress acts thereon even when the operation of the turbine is stopped, and further a centrifugal force is additionally imparted thereto when the turbine is in operation. Therefore the operational stress becomes large, with the result that the stress/corrosion cracking occurs depending solely upon the remaining two factors.
The tensile stress which is produced in the disc by reason of the shrinkage fit becomes large as the rotor temperature is low. Therefore a rise in rotational speed of the rotor despite low temperature, in particular, at the time of starting of the turbine, results in the addition of a tensile stress due to the centrifugal force acting on the disc and moving vanes to the tensile stress due to the shrinkage fit. In consequence, the crack is rapidly enlarged to produce the possibility that a serious accident wherein the disc breaks takes place.